This invention relates to a method for forming and consolidating fiber reinforced composite structures with hollow sections or passages through the structure.
Formation of high quality, complex, and dimensionally accurate hollow sections in resin based composite parts reinforced with such fibers as glass, quartz and carbon fibers has been limited by core material performance. In some cases plaster cores are too brittle for normal handling procedures. This can be a particular problem when thin core cross-sections are involved. Rubber has temperature limitations and cannot provide a dimensionally accurate hollow section. Metal cores such as steel or aluminum cannot be removed from complex shaped hollow sections under conditions mild enough to prevent excessive degradation of the resin based composite. Bi/Sn or Sn/Pb (commonly referred to as low melt alloys are known for creating relatively low melting point cores that in turn can be used to injection mold hollow plastic components. Additionally, these low melt alloys can be melted out to form very complex and dimensionally accurate passageways or hollow shapes without harming the physical properties of the plastic material. However, these low melting alloys are not useful as cores for polymer prepregs that cure at high temperatures in the range of from a minimum of about 600.degree. to a maximum of about 700.degree. F. and finally post-cure at temperatures from 700.degree. F. to about 750.degree. F.
In addition, fiber reinforced resin structures are formed into many complex shapes, in some cases these shapes include passageways which make consolidation of these structures during their cure cycle difficult. The structures are usually formed by wrapping a prepreg around one or more cores which are then attached to a steel mandrel and placed in an autoclave for curing under predetermined temperatures and pressures. What makes consolidation during the cure cycle so difficult is the formation of gaps between wrapped cores due to looseness required for assembly and the fact that the steel mandrel expands as the temperature increases which increases the gaps between the wrapped cores. In addition, volatiles are lost from the prepreg material as the temperature increases, thus decreasing the thickness of the prepreg material. All three of these factors contribute to a poorly consolidated product.